Method and System for Detecting Capture with Cancellation of Pacing Artifact

ABSTRACT

Methods and systems for detecting capture using pacing artifact cancellation are described. One or more pacing artifact templates are provided and a cardiac signal is sensed in a cardiac verification window. Each of the pacing artifact templates may characterize the pacing artifact associated with a particular pacing energy level, for example. A particular pacing artifact template is canceled from the cardiac signal. Capture is determined using the pacing artifact canceled cardiac signal. Detection of fusion/pseudofusion beats may be accomplished by comparing a cardiac signal to a captured response template.

RELATED PATENT DOCUMENTS

This application is a continuation of division of U.S. patentapplication Ser. No. 11/651,336 filed Jan. 9, 2007, which is a divisionof U.S. Pat. No. 7,162,301. This application claims priority to bothU.S. patent application Ser. No. 11/651,336 and U.S. Pat. No. 7,162,301which are incorporated herein by reference in their entireties.

FIELD OF THE INVENTION

The present invention relates generally to implantable medical devicesand, more particularly, to verifying capture in the heart by detectionof an evoked response following pacing.

BACKGROUND OF THE INVENTION

When functioning normally, the heart produces rhythmic contractions andis capable of pumping blood throughout the body. However, due to diseaseor injury, the heart rhythm may become irregular resulting in diminishedblood circulation. Arrhythmia is a general term used to describe heartrhythm irregularities arising from a variety of physical conditions anddisease processes. Cardiac rhythm management systems, such asimplantable pacemakers and cardiac defibrillators, have been used as aneffective treatment for patients with serious arrhythmias. These systemstypically comprise circuitry to sense electrical signals from the heartand a pulse generator for providing electrical pulses to the heart.Leads extending into the patient's heart are connected to electrodesthat contact the myocardium for sensing the heart's electrical signalsand for delivering pulses to the heart in accordance with varioustherapies for treating the arrhythmias.

Cardiac rhythm management systems operate to stimulate the heart tissueadjacent to the electrodes to produce a contraction of the tissue.Pacemakers are cardiac rhythm management systems that deliver a seriesof pace pulses to the heart. Pace pulses are typically low energyelectrical pulses timed to assist the heart in producing a contractilerhythm that maintains cardiac pumping efficiency. Pace pulses may beintermittent or continuous, depending on the needs of the patient. Thereexist a number of categories of pacemaker devices, with various modesfor sensing and pacing the heart.

When a pace pulse produces a contractile response in a heart, thecontractile response is typically referred to as capture, and theelectrical waveform corresponding to capture is denoted the evokedresponse. Superimposed with the evoked response may be a post paceresidual polarization waveform. The magnitude of the post pace residualpolarization waveform, denoted herein as the pacing artifact waveform,is affected by a variety of factors including lead polarization, afterpotential from the pace pulse, lead impedance, patient impedance, pacepulse width and pace pulse amplitude, for example.

A pace pulse must exceed a minimum energy value, or capture threshold,to produce a contraction. It is desirable for a pace pulse to havesufficient energy to stimulate capture of the heart without expendingenergy in excess of a capture threshold. Accurate determination of thecapture threshold is required for efficient pace energy management. Ifthe pace pulse energy is too low, the pace pulses may not reliablyproduce a contractile response in the heart resulting in ineffectivepacing. If the pace pulse energy is too high, the result may be patientdiscomfort as well as shorter battery life.

Capture detection allows the cardiac rhythm management system to adjustthe energy level of pace pulses to correspond to the optimum energyexpenditure that reliably produces a captured response. Further, capturedetection allows the cardiac rhythm management system to initiate aback-up pulse at a higher energy level whenever a pace pulse does notproduce a captured response.

A fusion beat is a cardiac contraction that occurs when two intrinsiccardiac depolarizations of a particular chamber, but from separateinitiation sites, merge. When the heart is being paced, a fusion beatmay occur when an intrinsic cardiac depolarization of a particularchamber merges with a pacer output pulse within that chamber. Fusionbeats, as seen on electrocardiographic recordings, exhibit variousmorphologies. The merging depolarizations of a fusion beat do notcontribute evenly to the total depolarization.

Pseudofusion occurs when a pacer output pulse artifact is superimposedupon a spontaneous P wave during atrial pacing, or upon a spontaneousQRS complex during ventricular pacing. In pseudofusion, the pacingstimulus is ineffective because the tissue around the electrode hasalready spontaneously depolarized and is in its refractory period.

During normal pacing, the presence of fusion and pseudofusion beats maybe of little consequence except for wasted energy due to the generationof unnecessary pace pulses. However, detection of fusion andpseudofusion beats may be required during an automatic capture orthreshold determination procedures. Fusion and pseudofusion beats maycause false detection of capture and may lead to erroneous capturethreshold values.

Capture may be verified by detecting a cardiac signal indicative of anevoked response. However, the evoked response must be discerned from thesuperimposed post pace residual polarization, denoted herein as a pacingartifact. In addition, fusion or pseudofusion beats may further obscurethe evoked response. It is desirable to detect the evoked response andthereby verify capture so that an effective pace pulse energy may bechosen and appropriate back up pacing delivered.

For the reasons stated above, and for other reasons stated below whichwill become apparent to those skilled in the art upon reading thepresent specification, there is a need in the art for a method anddevice that reliably and accurately detects capture in a patient's heartby sensing an evoked response in the presence of the post pace residualpolarization and possible fusion or pseudofusion beats. There exists afurther need for such an approach that is adaptive and accommodateschanges in the patient's capture threshold over time. The presentinvention fulfills these and other needs.

SUMMARY OF THE INVENTION

The present invention is directed to a method and device for detectingcapture using pacing artifact cancellation. In accordance with oneembodiment of the present invention, one or more pacing artifacttemplates are provided and a cardiac signal is sensed. A particularpacing artifact template is canceled from the cardiac signal. Capture isdetermined by analyzing the pacing artifact canceled cardiac signal.

In another embodiment of the invention, a method for detectingfusion/pseudofusion involves providing a captured response template. Acardiac signal is detected and the captured response template iscanceled from the cardiac signal to form a captured response canceledcardiac signal. Fusion/pseudofusion is determined using the capturedresponse canceled cardiac signal.

In a further embodiment of the invention, a medical device includes alead system extending into the heart. The lead system includeselectrodes positioned to detect cardiac signals. The lead system iscoupled to sensing circuitry to sense the cardiac signals. The devicefurther includes a pulse generator for producing pace pulses applied tothe heart through the electrodes. A control system controls theoperation of the device including the sense circuitry and the pulsegenerator. A capture detection system is coupled to the sensingcircuitry and is configured to provide pacing artifact templates, cancelthe pacing artifact template from the sensed cardiac signals, anddetermine if capture occurs using the pacing artifact canceled cardiacsignal.

In a further embodiment of the invention, a system for detecting capturein a patient's heart includes means for providing one or more pacingartifact templates, means for sensing a cardiac signal, means forcanceling a particular pacing artifact template from the cardiac signal,and means for determining if capture occurs using the pacing artifactcanceled cardiac signal.

Another embodiment of the invention involves a system for detectingfusion/pseudofusion in a patient's heart. The system includes means forproviding a captured response template, means for detecting a cardiacsignal, means for canceling the captured response template from thecardiac signal to form a captured response canceled cardiac signal, andmeans for detecting fusion/pseudofusion using the captured responsecanceled cardiac signal.

The above summary of the present invention is not intended to describeeach embodiment or every implementation of the present invention.Advantages and attainments, together with a more complete understandingof the invention, will become apparent and appreciated by referring tothe following detailed description and claims taken in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial view of one embodiment of an implantable medicaldevice with an endocardial lead system extending into atrial andventricular chambers of a heart;

FIG. 2 is a block diagram of an implantable medical device with whichcapture verification and fusion/pseudofusion detection in accordancewith the present invention may be implemented;

FIG. 3 is a flowchart conceptually illustrating a method of detectingcapture in accordance with an embodiment of the present invention;

FIG. 4 is a flowchart illustrating a method of forming a pacing artifacttemplate in accordance with an embodiment of the present invention;

FIG. 5 is a flowchart illustrating a method predicting a pacing artifacttemplate as an exponentially decaying function in accordance with anembodiment of the present invention;

FIG. 6 is a graph comparing a measured pacing artifact with a pacingartifact predicted as an exponentially decaying function;

FIG. 7 is a flowchart illustrating a method of detecting capture on abatch basis in accordance with an embodiment of the present invention;

FIG. 8A is a graph illustrating normalization of a pacing artifacttemplate in accordance with an embodiment of the invention;

FIG. 8B is a graph comparing a captured evoked response template to anormalized pacing artifact template;

FIG. 9 is a graph comparing a captured and non-captured waveform in acapture verification window;

FIG. 10 is a flowchart of a method of detecting capture on a beat bybeat basis in accordance with an embodiment of the present invention;

FIG. 11 is a flowchart conceptually illustrating a method of detectingfusion/pseudofusion in accordance with an embodiment of the presentinvention;

FIGS. 12A-12D are graphs illustrating a method fusion/pseudofusiondetection window in accordance with an embodiment of the invention; and

FIG. 13 is a flowchart conceptually illustrating a method of detectingcapture and fusion/pseudofusion in accordance with an embodiment of thepresent invention;

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail below. It is to be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the invention isintended to cover all modifications, equivalents, and alternativesfalling within the scope of the invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

In the following description of the illustrated embodiments, referencesare made to the accompanying drawings forming a part hereof, and inwhich are shown by way of illustration, various embodiments by which theinvention may be practiced. It is to be understood that otherembodiments may be utilized, and structural and functional changes maybe made without departing from the scope of the present invention.

The embodiments of the present system illustrated herein are generallydescribed as being implemented in an implantable cardiac defibrillator(ICD) that may operate in numerous pacing modes known in the art.Various types of single and multiple chamber implantable cardiacdefibrillators are known in the art and may implement a captureverification methodology of the present invention. Furthermore, thesystems and methods of the present invention may also be implemented insingle and multi chamber pacemakers, resynchronizers, andcardioverter/monitor systems, for example.

Although the present system is described in conjunction with animplantable cardiac defibrillator having a microprocessor-basedarchitecture, it will be understood that the implantable cardiacdefibrillator (or other device) may be implemented in any logic-basedintegrated circuit architecture, if desired.

The present invention provides a system and method for monitoring apatient's electrocardiogram and verifying that capture occurs followingapplication of a pace pulse. Capture detection, along withfusion/pseudofusion detection, may be used, for example, in connectionwith an automatic capture verification procedure and the determinationof the optimal energy of a pace pulse, such as in an automatic capturethreshold procedure. Additionally, capture verification may be used on abeat-by-beat basis to control back-up pacing initiated when a pace pulsedelivered to the heart fails to evoke a contractile response. These andother applications may be enhanced by employment of the systems andmethods of the present invention.

Those skilled in the art will appreciate that reference to a capturethreshold procedure indicates a method of measuring the stimulationthreshold in either an atrium or a ventricle. In such a procedure, thepacemaker, automatically or upon command, initiates a search for thecapture threshold of the chamber. In one example of an automatic capturethreshold procedure, the pacemaker automatically decreases the pulseamplitude in discrete steps until a predetermined number of consecutiveloss-of-capture events occur. At that point, the pacemaker may increasethe stimulation voltage in discrete steps until a predetermined numberof capture events occur to confirm the capture threshold. Variousmethods of implementing capture threshold procedures are known in theart and may be enhanced by the capture detection methods of the presentinvention.

Automatic capture threshold determination is distinguishable fromautomatic capture detection which is a procedure that occurs on abeat-by-beat basis. Automatic capture detection confirms on abeat-by-beat basis that a delivered pace pulse results in an evokedresponse. When no evoked response is detected following a pace pulse,the pacemaker may deliver a back up safety pulse to ensure consistentpacing. If a predetermined number of pace pulses delivered during normalpacing do not produce an evoked response, the pacemaker may initiate acapture threshold test as described above. The various procedures forimplementing automatic capture detection and/or back up pacing processesmay be enhanced by the capture detection methods described herein.

According to the present invention, a template of the pacing artifact isacquired or predicted by various methods. The pacing artifact templateis canceled from a cardiac signal sensed following a pace pulse. Captureis determined using the pacing artifact canceled cardiac signal usingvarious techniques. In one embodiment, capture detector circuitrydetermines capture has occurred by comparing an amplitude of the pacingartifact canceled cardiac signal to an amplitude associated with anevoked response. By this method, the pacing artifact canceled cardiacsignal in a specified time window following the stimulation pulse isexamined to determine if capture has occurred. If the pacing artifactcanceled cardiac signal achieves the amplitude associated with an evokedresponse, the capture detector circuitry determines that capture hasoccurred.

In other embodiments, the capture detector may detect features of apacing artifact canceled cardiac signal consistent with the morphologyof an evoked response to determine capture. An exemplary set of featuresthat may be used to determine capture include a slope of the cardiacsignal, timing of local maxima or minima of the cardiac signal, or therise time and/or fall times of the pacing artifact canceled cardiacsignal. Other features of the pacing artifact canceled cardiac signalmay also be used to determine if capture has occurred.

During a capture verification procedure, it may also be desirable todetect fusion and pseudofusion beats to prevent false capture detection.A method of detecting fusion/pseudofusion beats in accordance with thepresent invention relies upon canceling a template representative of acaptured response from a sensed cardiac waveform and examining theresultant waveform. The captured response template includes twosuperimposed component signals, an evoked response and a pacing artifactresponse. The evoked response component represents the cardiac signalassociated with contraction of the heart tissue in response to the pacepulse. The pacing artifact component represents the post pace residualpolarization waveform. A fusion/pseudofusion beat may be discriminatedfrom a captured response beat based on beat waveform morphologycharacteristics, as described in connection with capture detectionabove. For example, a fusion/pseudofusion beat may have a larger peakamplitude when compared to a captured response, allowing thefusion/pseudofusion beat to be detected.

Referring now to FIG. 1 of the drawings, there is shown one embodimentof a cardiac rhythm management system that includes an implantablecardiac defibrillator 100 electrically and physically coupled to anintracardiac lead system 102. The intracardiac lead system 102 isimplanted in a human body with portions of the intracardiac lead system102 inserted into a heart 101. The intracardiac lead system 102 is usedto detect and analyze electrical cardiac signals produced by the heart101 and to provide electrical energy to the heart 101 under certainpredetermined conditions to treat cardiac arrhythmias, including, forexample, ventricular fibrillation of the heart 101.

The intracardiac lead system 102 includes one or more pacing electrodesand one or more intracardiac defibrillation electrodes. In theparticular embodiment shown in FIG. 1, the intracardiac lead system 102includes a ventricular lead system 104 and an atrial lead system 106.The ventricular lead system 104 includes an SVC-coil 116, an RV-coil114, and an RV-tip electrode 112. The RV-coil 114, which mayalternatively be an RV-ring electrode, is spaced apart from the RV-tipelectrode 112, which is a pacing electrode. In one embodiment, theventricular lead system 104 is configured as an integrated bipolarpace/shock lead. In another exemplary configuration, one or moreadditional electrodes, e.g., a ring electrode, may be included in theventricular lead system 104. The additional ring electrode and theRV-tip electrode 112 may be used for bipolar sensing of cardiac signals.The atrial lead system 106 includes an A-tip electrode 152 and an A-ringelectrode 154. In one embodiment, the atrial lead system 106 isconfigured as an atrial J lead.

In this configuration, the intracardiac lead system 102 is positionedwithin the heart 101, with portions of the atrial lead system 106extending into the right atrium 120 and portions of the ventricular leadsystem 104 extending into the right atrium 120 and right ventricle 118.In particular, the A-tip electrode 152 and A-ring electrode 154 arepositioned at appropriate locations within the right atrium 120. TheRV-tip electrode 112 and RV-coil 114 electrodes are positioned atappropriate locations within the right ventricle 118. The SVC-coil 116is positioned at an appropriate location within the right atrium chamber120 of the heart 101 or a major vein leading to the right atrium chamber120 of the heart 101. The RV-coil 114 and SVC-coil 116 depicted in FIG.1 are defibrillation electrodes.

Additional pacing and defibrillation electrodes may also be included inthe intracardiac lead system 102 to allow for various bipolar sensing,pacing, and defibrillation capabilities. For example, the intracardiaclead system 102 may include endocardial pacing andcardioversion/defibrillation leads (not shown) that are advanced intothe coronary sinus and coronary veins to locate the distal electrode(s)adjacent to the left ventricle or the left atrium. Other intracardiaclead and electrode arrangements and configurations known in the art arealso possible and considered to be within the scope of the presentsystem.

The ventricular and atrial lead systems 104, 106 include conductors forcommunicating sense, pacing, and defibrillation/cardioverter signalsbetween the cardiac defibrillator 100 and the electrodes and coils ofthe lead systems 104, 106. As is shown in FIG. 1, ventricular leadsystem 104 includes a conductor 108 for transmitting sense and pacingsignals between the RV-tip electrode 112 and an RV-tip terminal 202within the cardiac defibrillator 100. A conductor 110 of the ventricularlead system 104 transmits sense signals between the RV-coil or ringelectrode 114 and an RV-coil terminal 204 within the cardiacdefibrillator 100. The ventricular lead system 104 also includesconductors 122 for transmitting sense and defibrillation signals betweenterminal 206 of the cardiac defibrillator 100 and the SVC-coil 116. Theatrial lead system 106 includes conductors 132, 134 for transmittingsense and pacing signals between terminals 212, 210 of the cardiacdefibrillator 100 and A-tip and A-ring electrodes 152 and 154,respectively.

Referring now to FIG. 2, there is shown an embodiment of a cardiacdefibrillator 200 suitable for implementing a capture verificationmethodology of the present invention. FIG. 2 shows a cardiacdefibrillator divided into functional blocks. It is understood by thoseskilled in the art that there exist many possible configurations inwhich these functional blocks can be arranged. The example depicted inFIG. 2 is one possible functional arrangement. The cardiac defibrillator200 includes circuitry for receiving cardiac signals from a heart 101(not shown in FIG. 2) and delivering electrical energy to the heart. Thecardiac defibrillator 200 includes terminals for connecting the cardiacdefibrillator 200 to the electrodes of the intracardiac lead system aspreviously discussed.

In one embodiment, the cardiac defibrillator circuitry 203 of thecardiac defibrillator 200 is encased and hermetically sealed in ahousing 201 suitable for implanting in a human body as is known in theart. Power to the cardiac defibrillator 200 is supplied by anelectrochemical battery 233 that is housed within the cardiacdefibrillator 200. A connector block (not shown) is additionallyattached to the housing 201 of the cardiac defibrillator 200 to allowfor the physical and electrical attachment of the intracardiac leadsystem conductors to the cardiac defibrillator 200 and the encasedcardiac defibrillator circuitry 203.

In one embodiment, the cardiac defibrillator circuitry 203 of thecardiac defibrillator 200 is a programmable microprocessor-based system,including a control system 220 and a memory circuit 236. The memorycircuit 236 stores parameters for various pacing, defibrillation, andsensing modes and stores data indicative of cardiac signals received byother components of the cardiac defibrillator circuitry 203. The controlsystem 220 and memory circuit 236 cooperate with other components of thecardiac defibrillator circuitry 203 to perform operations involving thecapture verification according to the principles of the presentinvention, in addition to other sensing, pacing and defibrillationfunctions. The control system 220 may encompass additional functionalcomponents including a pacemaker 222, an arrhythmia detector 240 andtemplate generator 241 along with other functions for controlling thecardiac defibrillator circuitry 203. A memory 232 is also provided forstoring historical EGM and therapy data. The historical data may be usedfor various purposes to control the operations of the cardiacdefibrillator 200 and may also be transmitted to an external programmerunit 234 as needed or desired.

Telemetry circuitry 231 is additionally coupled to the cardiacdefibrillator circuitry 203 to allow the cardiac defibrillator 200 tocommunicate with an external programmer unit 234. In one embodiment, thetelemetry circuitry 231 and the programmer unit 234 use a wire loopantenna and a radio frequency telemetric link, as is known in the art,to receive and transmit signals and data between the programmer unit 234and telemetry circuitry 231. In this manner, programming commands may betransferred to the control system 220 of the cardiac defibrillator 200from the programmer unit 234 during and after implant. In addition,stored cardiac data pertaining to capture verification and capturethreshold, along with other data, may be transferred to the programmerunit 234 from the cardiac defibrillator 200, for example.

Cardiac signals sensed through use of the RV-tip electrode 112 arenear-field signals or rate channel signals as are known in the art. Moreparticularly, a rate channel signal is detected as a voltage developedbetween the RV-tip electrode 112 and the RV-coil 114. Cardiac signalssensed through use of one or both of the defibrillation coils orelectrodes 114, 116 are far-field signals, also referred to asmorphology or shock channel signals, as are known in the art. Moreparticularly, a shock channel signal is detected as a voltage developedbetween the RV-coil 114 and the SVC-coil 116. A shock channel signal mayalso be detected as a voltage developed between the RV-coil 114 and thecan electrode 209. Alternatively, the can electrode 209 and the RV-coilelectrode may be shorted and a shock channel signal sensed as thevoltage developed between the RV-coil 114 and the can electrode 209 andSVC-coil 116 combination. Shock channel signals developed usingappropriate combinations of the RV-coil, SVC-coil, and can electrodes114, 116 and 209 are sensed and amplified by a shock EGM amplifier 229.The output of the EGM amplifier 229 is coupled to the control system220.

In the embodiment of the cardiac defibrillator 200 depicted in FIG. 2,RV-tip and RV-coil electrodes 112, 114 are shown coupled to a V-senseamplifier 226 and thus to an R-wave detector 223. Rate channel signalsreceived by the V-sense amplifier 226 are communicated to the R-wavedetector 223, which serves to sense and amplify the rate channelsignals, e.g. R-waves. The sensed R-waves may then be communicated tothe control system 220.

A-tip and A-ring electrodes 152, 154 are shown coupled to an A-senseamplifier 225. Atrial sense signals received by the A-sense amplifier225 are communicated to an A-wave detector 221, which serves to senseand amplify the A-wave signals. The atrial signals may be communicatedfrom the A-wave detector 221 to the control system 220.

The pacemaker 222 communicates pacing signals to the RV-tip and A-tipelectrodes 112 and 152 according to a preestablished pacing regimenunder appropriate conditions. Blanking circuitry (not shown) is employedin a known manner when a ventricular or atrial pacing pulse isdelivered, such that the ventricular channel, atrial channel, and shockchannel are properly blanked at the appropriate time and for theappropriate duration.

A switching matrix 228, shown in FIG. 2, may be coupled to the A-tip152, A-ring 154, RV-tip 112, RV-coil 114 and SVC-coil 116 electrodes.The switching matrix 228 may provide connections to variousconfigurations of pacing and defibrillation electrodes, for example. Theoutputs of the switching matrix 228 are coupled to an ER amplifier 227which serves to sense and amplify signals detected between the selectedcombinations of electrodes. The detected signals are coupled through theER amplifier to a capture detector 224. The capture detector 224includes circuitry configured to detect an evoked response and verifycapture in accordance with the invention. The capture detector furtherincludes circuitry for detecting fusion/pseudofusion beats.

FIG. 3 is a flowchart illustrating various processes for captureverification according to an embodiment of the present invention.According to the embodiment illustrated in the flowchart of FIG. 3, uponcommencement of capture verification, a pacing artifact templaterepresentative of post-pace residual polarization is provided 310. Aftera pace pulse is delivered, a resultant cardiac signal is sensed 320. Thepacing artifact template is canceled from the sensed cardiac signal 330.Capture is determined using the pacing artifact canceled cardiac signal340.

FIG. 4 illustrates a method of generating a pacing artifact templateaccording to another embodiment of the present invention. The methoddescribed in the following paragraph, with reference to FIG. 4, may beused to provide a pacing artifact template at blocks 310, 720, 1010, and1305 in FIGS. 3, 7, 10, and 13, respectively. In the exemplaryembodiment illustrated by FIG. 4, a pacing artifact template is formedthrough an iterative process. A number of pace pulses are delivered 410to generate pacing artifact waveforms. The pace pulses are delivered insuch a way that capture does not occur. The resultant cardiac signalrepresents a pure pacing artifact waveform without a superimposed evokedresponse. Pacing artifact signals without an associated evoked responsemay be produced by delivering 411 pace pulses at an energy level lowerthan the pacing threshold. Alternatively, the pace pulses may bedelivered 412 during a myocardial refractory period. The myocardialrefractory period represents a time when the heart tissue is recoveringfrom a previous cardiac beat. Pace pulses delivered during themyocardial refractory period typically do not produce an evoked responsein the heart tissue, thus a pure pacing artifact waveform may beacquired.

Following generation 410 of a pace pulse using either of the abovemethods described in connection with blocks 411 or 412, a pacingartifact waveform is sensed 420 during a cardiac verification window.For example, the cardiac verification window may begin approximately 25ms after delivery of the pace pulse and continue for a time interval ofapproximately 50 ms. The pacing artifact waveform is averaged withpreviously acquired pacing artifact waveforms 430, if any. The processof generating a pace pulse and detecting the resultant pacing artifactwaveform 410-430 is repeated until a predetermined number of pacingartifact waveforms has been acquired 440. When a sufficient number ofpacing artifact waveforms has been acquired 440, the average pacingartifact waveform is stored 450 as the pacing artifact template.

The pacing artifact may exhibit small variations in morphology withrespect to pace pulse amplitude. Accordingly, the use of multiple pacingartifact templates corresponding to various pace pulse amplitudes mayprovide a more thorough cancellation of the pacing artifact over a rangeof pace pulse amplitudes, e.g., as used in a pacing threshold test. Themethod illustrated in FIG. 4 can be applied to generate pacing artifacttemplates for each pacing pulse amplitude of interest.

Alternatively, or additionally, a set of two or more pacing artifacttemplates may be generated, wherein a particular pacing artifacttemplate characterizes the pacing artifact associated with a small rangeof pace pulse amplitudes. A pacing artifact template for a pace pulserange can be formed by combining pacing artifact waveforms from variouspace pulse amplitudes within the range using, for example, an averagingoperation. The pacing artifact template for a pace pulse range may alsobe formed by selecting a pacing artifact waveform at a single pace pulseamplitude, e.g., a pacing artifact waveform for a pulse amplitude nearthe center of the range to be characterized. The set of pacing artifacttemplates correspond to the entire pace pulse amplitude range to beevaluated.

The artifact waveform measurement may be accomplished during therefractory period of the myocardium. Pace pulses delivered during therefractory period produce pacing artifact waveforms without the evokedresponse components. The timing of the pace pulse delivered for pacingartifact measurement in the myocardial refractory period should beselected to be before the vulnerable period of the myocardium to avoidpro-arrhythmia, and after the deflections from the myocardial responsefrom the previous cardiac event in the chamber have passed, e.g., 80 msafter the preceding cardiac event.

FIG. 5 illustrates an alternative method of providing a pacing artifacttemplate according to an embodiment of the present invention. The methoddescribed in the following paragraph, with reference to FIG. 5, may beused to provide a pacing artifact template at blocks 310, 720, 1010 and1305 in FIGS. 3, 7, 10, and 13, respectively. A typical pacing artifactwaveform in the cardiac verification window has a shape that can beapproximated by an exponentially decaying function associated with aparticular time constant. The time constant of an average pacingartifact waveform may be estimated and the pacing artifact templatepredicted using the estimated time constant.

In this exemplary embodiment, an average pacing artifact waveform isdetermined by sensing a predetermined number of pacing artifactwaveforms generated by a predetermined number of pace pulses. Aspreviously discussed, each pace pulse is delivered in such a way thatcapture does not occur, resulting in a pure pacing artifact waveformwithout a superimposed evoked response. The pace pulses may be delivered511 at an energy level lower than the pacing threshold, for example.Alternatively, the pace pulses may be delivered 512 during a myocardialrefractory period when the pace pulses cannot produce an evokedresponse.

Following delivery 510 of a pulse, the pacing artifact waveform issensed 520 during a cardiac verification window. The pacing artifactwaveform is averaged 530 with previously acquired pacing artifactwaveforms. The process 510-530 of delivering a pulse and detecting theresultant pacing artifact waveform is repeated until the predeterminednumber of pacing artifact waveforms has been acquired 540.

When a sufficient number of pacing artifact waveforms has been acquired540, a time constant of the average pacing artifact waveform isestimated 550. The pacing artifact template is predicted 560 as anexponentially decaying function using the estimated time constant. Apacing artifact template generated by either of the methods described inthe preceding paragraphs with reference to FIGS. 4 and 5 may beperiodically updated by acquiring additional pacing artifact waveformsand combining the additional pacing artifact waveforms with the pacingartifact template. For example, the additional pacing artifact waveformsmay be combined with the pacing artifact template by averaging.

FIG. 6 shows a comparison of the actual pacing artifact template graph610 and the pacing artifact template predicted as an exponentiallydecaying function 620. The pacing artifact template can be predicted fora sampled signal using Equation (1):

x(t)=A*x(t−1)  (1)

-   -   where A=e^(−T/a)        where x(t) represents a current sample of the pacing artifact        template, x(t−1) represents a previous sample of the pacing        artifact template, and A is a constant derived from the        estimated time constant of the pacing artifact template, a, and        the sample time, T.

According to one embodiment, a signal corresponding to the pacingartifact template may be generated by hardware using digital circuitryto produce the function of Equation (1). The pacing artifact templatesignal generated in hardware may be used to cancel the pacing artifactfrom a sensed cardiac signal in the capture verification window. Inother embodiments, the pacing artifact template may be canceled from thesensed cardiac signal using software-based techniques. For example, thepacing artifact template may be canceled by subtracting stored values ofthe template from the sensed cardiac signal at each sample point.

Although the examples provided herein predict the pacing artifact usingan exponential function, those skilled in the art will recognize thatprediction of the pacing artifact is not limited to characterization byan exponential function. Any function or combination of functions may beused to characterize and predict the pacing artifact.

FIG. 7 illustrates a method of detecting capture by a batch process inaccordance with an embodiment of the invention. A predetermined numberof cardiac signals sensed following pace pulses are stored 710 inmemory. Each cardiac signal is sensed during the cardiac verificationwindow following the delivery of a pace pulse. The cardiac signals mayrepresent a series of signals responsive to ramping down the pace pulseenergy in a capture threshold determination procedure, for example. Apacing artifact template may be provided 720 by any of the methodspreviously discussed in connection with FIG. 4 or FIG. 5.

A previously stored cardiac signal is retrieved 730 from memory. Thepacing artifact template is normalized 735 using one or more samples ofthe cardiac signal. In one example, one or more samples of the cardiacsignal are averaged and the pacing artifact template normalized withrespect to the average value. In another example, a representative setof the cardiac signal samples may be used to define a slope of thecardiac signal within the cardiac verification window. The pacingartifact template may be normalized with respect to a point extrapolatedusing the slope.

Normalization of the pacing artifact template with respect to a firstsample of the cardiac signal in the capture verification window isillustrated in FIGS. 8A and 8B. FIG. 8A shows a graph of a pacingartifact template before normalization 810 and after normalization 820with respect to a sensed cardiac signal 830. FIG. 8B shows the graph ofa normalized pacing artifact template 840 overlaying the graph of thesensed cardiac signal 830.

Returning now to FIG. 7, following normalization 735 of the pacingartifact template with respect to the sensed cardiac signal, thenormalized pacing artifact template is canceled 740 from the cardiacsignal. Capture is determined by analyzing the pacing artifact canceledcardiac signal 750. If additional cardiac signal waveforms remain to beprocessed for capture verification 760, the process described at blocks730-750 is repeated until all of the cardiac signals have been processedand the capture verification process is complete 770 for all waveforms.

Cancellation of a pacing artifact template from a sensed cardiac signalin accordance with the invention is illustrated in FIG. 9. A captureverification window commences approximately 25 ms following delivery ofthe pacing pulse and extends for approximately 50 ms. The pacingartifact template 920 is shown overlaying the sensed cardiac waveform ofa captured beat 930. Cancellation of the pacing artifact template 920from the sensed cardiac waveform 930 results in a pacing artifactcanceled waveform 940 from which capture is determined. In the exampleof FIG. 9, capture is indicated by the presence of the local minima 950at approximately 53 ms following the pace pulse.

Turning now to FIG. 10, a method of detecting capture on a sample bysample basis in accordance with an embodiment of the invention isillustrated. A pacing artifact template is provided 1010 by either ofthe methods previously discussed in connection with FIGS. 4 and 5. Acardiac signal following a pace pulse is sampled 1020. The pacingartifact template is normalized 1025 with respect to the first sample ofthe cardiac signal. The pacing artifact template is canceled 1030 fromthe cardiac signal at each sample point. The loop described by blocks1020-1030 continues until all samples of the cardiac signal have beenprocessed 1040 or until capture is detected 1035 and captureverification is complete 1050.

As previously discussed, fusion or pseudofusion may occur during pacing.A fusion beat occurs when an intrinsic cardiac depolarization of aparticular chamber merges with a pacer output pulse within that chamber.Pseudofusion occurs when a pacer output pulse artifact is superimposedupon a spontaneous P wave during atrial pacing, or upon a spontaneousQRS complex during ventricular pacing. In pseudofusion, the pacingstimulus is ineffective because the tissue around the electrode hasalready spontaneously depolarized and is in its refractory period.During a capture verification procedure, it may be desirable to detectfusion and pseudofusion beats to prevent false capture detection.

A method of detecting fusion/pseudofusion beats in accordance with thepresent invention relies upon canceling a template representative of acaptured response from a sensed cardiac waveform and examining theresultant waveform. The captured response template includes twosuperimposed component signals, an evoked response and a pacing artifactresponse. The evoked response component represents the cardiac signalassociated with contraction of the heart tissue in response to the pacepulse. The pacing artifact component represents the post pace residualpolarization waveform. A fusion/pseudofusion beat may be discriminatedfrom a captured response beat based on beat waveform morphologycharacteristics. For example, a fusion/pseudofusion beat may have alarger peak amplitude when compared to a captured response, allowing thefusion/pseudofusion beat to be detected.

A method of fusion/pseudofusion detection in accordance with anembodiment of the present invention is illustrated in FIG. 11. Acaptured response template is provided 1110 which is representative of acaptured beat waveform within a fusion/pseudofusion detection window.The fusion/pseudofusion detection window may begin, for example, at theend of a blanking period and extend for approximately 20 ms. A blankingperiod of approximately 10 ms may follow a pace pulse during whichsensing is inhibited to prevent erroneous sensing of a cardiac response.A cardiac waveform is sensed 1120 in the fusion/pseudofusion detectionwindow following a pace pulse. The captured response template iscanceled 1130 from the sensed cardiac waveform. Fusion/pseudofusion isdetermined 1140 by analyzing the waveform resulting from thecancellation of the captured response template from the sensed cardiacwaveform.

Providing a captured response template may also encompass periodicallyupdating the captured response template. The captured response templatemay be periodically updated by averaging a captured response waveformwith the existing captured response template.

FIGS. 12A-D are graphs illustrating detection of a pseudofusion beat inaccordance with an embodiment of the invention. FIGS. 12A and 12Billustrate a graph of a captured response template withoutfusion/pseudofusion, and the graph of a pseudofusion beat, respectively.FIGS. 12C and 12D illustrate the captured response signal and apseudofusion beat after cancellation of the captured response template.Pseudofusion may be detected by comparing the captured response templatecanceled captured beat in FIG. 12C and the captured response templatecanceled pseudofusion beat in FIG. 12D in the fusion/pseudofusiondetection window 1210.

FIG. 13 is a flowchart illustrating capture verification, includingfusion/pseudofusion detection in accordance with the present invention.A pacing artifact template is provided 1305, for example, by one of themethods discussed in the previous paragraphs with reference to FIGS. 4and 5. Following generation of a pace pulse, a cardiac signal is sensed1310 during a cardiac verification window. The pacing artifact templateis normalized 1312 with respect to the cardiac signal and the pacingartifact template is canceled 1315 from the cardiac signal. Capture isdetermined 1320 using the pacing artifact canceled cardiac signal.

A captured response template is provided 1325 that is representative ofa captured beat. The captured response template is canceled 1330 fromthe cardiac signal in the fusion/pseudofusion detection window andfusion/pseudofusion is determined 1335 from analysis of the resultantwaveform.

Various modifications and additions can be made to the preferredembodiments discussed hereinabove without departing from the scope ofthe present invention. Accordingly, the scope of the present inventionshould not be limited by the particular embodiments described above, butshould be defined only by the claims set forth below and equivalentsthereof.

1. A method for operating a medical device, comprising: deliveringnon-capturing electrical stimulation pulses to a heart at a plurality ofpulse amplitudes; sensing pacing artifact waveforms associated with eachpulse amplitude of the plurality of pulse amplitudes; forming pacingartifact templates associated with the plurality of pulse amplitudes;delivering a subsequent electrical stimulation pulse to the heart;sensing a cardiac signal following the subsequent electrical stimulationpulse; canceling a pacing artifact template of the plurality of pacingartifact templates from the sensed cardiac signal to form a pacingartifact template canceled signal; and detecting capture of the heartusing the pacing artifact template canceled signal.
 2. The method ofclaim 1, wherein the pacing artifact template that is canceled from thecardiac signal is associated with a pulse amplitude.
 3. The method ofclaim 1, wherein the pacing artifact template that is canceled from thecardiac signal is associated with range of pulse amplitudes.
 4. Themethod of claim 1, wherein forming the pacing artifact templatescomprises combining pacing artifact waveforms associated with a range ofpulse amplitudes.
 5. The method of claim 1, wherein forming the pacingartifact templates comprises forming a set of pacing artifact templates,the set of pacing artifact templates corresponding to a pulse amplituderange to be evaluated, each pacing artifact template of the setassociated with a smaller range of pulse amplitudes within the pulseamplitude range to be evaluated.
 6. The method of claim 1, whereindelivering the non-capturing electrical stimulation pulses comprisesdelivering the non-capturing stimulation pulses while the myocardium isrefractory.
 7. The method of claim 1, wherein detecting capture of theheart using the pacing artifact template canceled signal comprisescomparing the pacing artifact cancelled cardiac signal to an amplitudereference characterizing an evoked response.
 8. The method of claim 1,wherein detecting capture of the heart using the pacing artifacttemplate canceled signal comprises comparing the pacing artifactcancelled cardiac signal to a morphology template characterizing anevoked response.
 9. The method of claim 1, wherein forming the pacingartifact templates comprises: averaging the pacing artifact waveforms toform an average pacing artifact waveform; and estimating a time constantfrom the average pacing artifact waveform.
 10. The method of claim 1,further comprising periodically updating the pacing artifact templates.11. The method of claim 1, wherein each of the pacing artifact templatesis formed as an exponential function or a combination of exponentialfunctions.
 12. A medical device, comprising: electrodes configured toelectrically couple to a heart; pulse generator circuitry coupled to theelectrodes and configured to provide stimulation pulses to the heart ata plurality of pulse amplitudes; sensing circuitry configured to sensecardiac electrical signals following the stimulation pulses; templatecircuitry configured to form pacing artifact templates associated withthe plurality of pulse amplitudes; and a capture detector configured tocancel a pacing artifact template of the plurality of pacing artifacttemplates from a cardiac signal sensed following a stimulation pulse andto detect capture of the heart using the pacing artifact cancelledcardiac signal.
 13. The medical device of claim 12, wherein the pacingartifact template canceled from the cardiac signal is associated with arange of pulse amplitudes.
 14. The medical device of claim 13, whereinthe pacing artifact template is associated with a pulse amplitude near acenter of the range of pulse amplitudes.
 15. The medical device of claim12, wherein forming the pacing artifact templates comprises combiningpacing artifact waveforms associated with a range of pulse amplitudes.16. The medical device of claim 12, wherein forming the pacing artifacttemplates comprises forming a set of pacing artifact templates, the setof pacing artifact templates corresponding to a pulse amplitude range tobe evaluated, each pacing artifact template of the set associated with asmaller range of pulse amplitudes within the pulse amplitude range to beevaluated.
 17. The medical device of claim 12, wherein the capturedetector is configured to detect capture by comparing the pacingartifact cancelled cardiac signal to an amplitude referencecharacterizing an evoked response.
 18. The medical device of claim 12,wherein the capture detector is configured to detect capture bycomparing the pacing artifact cancelled cardiac signal to a morphologytemplate characterizing an evoked response.
 19. The medical device ofclaim 12, wherein the template circuitry is configured to average pacingartifact waveforms to form an average pacing artifact waveform, estimatea time constant from the average pacing artifact waveform, and form apacing artifact template associated with a pacing pulse amplitude usingthe estimated time constant.
 20. A medical device, comprising:electrodes configured to electrically couple to the heart; pulsegenerator circuitry coupled to the electrodes and configured to providestimulation pulses at a plurality of pulse amplitudes to the heart viathe electrodes; sensing circuitry configured to sense cardiac electricalsignals following the stimulation pulses; means for forming pacingartifact templates associated with the plurality of pulse amplitudes;means for canceling a pacing artifact template of the plurality ofpacing artifact templates from a cardiac signal sensed following astimulation pulse and detecting capture of the heart using the pacingartifact cancelled cardiac signal.